Bike size calculator - Measure - model - analyse - change...

Bicycle geometry

What are the key geometric metrics?

Also, 'Bicycling Science' provides just about everything you could want to know about the history of bicycles,
how human beings propel them, what makes them go faster, and what keeps them from going even faster.
The scientific and engineering information is of interest not only to designers and builders of bicycles
and other human-powered vehicles but also to competitive cyclists, bicycle commuters, and recreational cyclists.

The third edition begins with a brief history of bicycles and bicycling that demolishes many widespread myths.
This edition includes information on recent experiments and achievements in human-powered transportation,
including the 'ultimate human-powered vehicle', in which a supine rider in a streamlined enclosure steers by
looking at a television screen connected to a small camera in the nose, reaching speeds of around 80 miles per hour.

I have used 'Bicycling Science' as a reference for some of the formulas used in this website.

It contains completely new chapters on aerodynamics, unusual human-powered machines for use on land and in water and air, human physiology, and the future of bicycling.
This edition also provides updated information on rolling drag, transmission of power from rider to wheels, braking, heat management,
steering and stability, power and speed, and materials.

Trail

One of the most important dimensions is trail. The virtual line where the steering axis intersects the ground is ahead of the tyres contact point with the ground.
As the bike moves forward this gives the wheel a self centering 'castoring' effect rather like a shopping trolley wheel.
In simple terms, the bigger the trail the more stable it is but the more reluctant it will be to turn.

I have experimented with different 'trail' lengths on motocross bikes. You can vary the trail on these bikes with offset axles and different offset triple clamps.

If the trail is too small the bike turns quickly at relatively slow speeds round posts / trees. It feels like you are turning the bike rather than leaning it.
As you speed up and you want to lean the bike such that it smoothly arcs through a wide fast bend it won't. It tries to turn rather than lean and the tyres seem to scrub.
Things seem very unstable.

If the trail is too long it is very stable at speed and leans and curves through a graceful arc. When you try and flick it round a corner, it won't!
It leans a bit and just goes nearly straight on

Somewhere in the middle it's just perfect!

On a bicycle with front suspension, when you hit a bump, brake hard or go down a hill fast and get to the bottom the forks compress to absorb the shock.
As they compress the trail is reduced and the bike becomes less stable. In most cases this is undesirable. These are just the times when you don't want an unstable bike.
I have observed someone getting to the bottom of a steep hill on a full suspension bike and the handlebars shaking violently. This is likely to be due to the reduced trail.
The cure is to fit longer forks and increase the trail all round or fit harder springs / put more air in the forks.
What are reasonable figures for trail? Go to Create a map and you can load the geometry for different bikes:

With my Giant Trance (XL) with 120mm forks the model shows that the trail goes from 88mm when uncompressed to 52mm when compressed.

With my Specialized Hardrock Sport (XL) with 100mm forks the model shows that the trail goes from 86mm when uncompressed to 55mm when compressed.

With my Giant Defy (XL) roadbike with no suspension the model shows that the trail is 78mm.

I will continue to model other bikes, but if these figures are representative, it would suggest that the norm would be a minimum of 50mm and max of 90mm for trail.

Note that motocrossers use this reduced trail to good effect when cornering. One effect of braking hard at the corner and shifting their weight forward is reduced trail.
The reduced trail makes the bike turn quicker. As they exit the corner they sit further back, unloading the front of the bike, lengthening the forks and increasing the trail.

When you change forks on a bicycle if the length of the forks or the offset between the steering axis and axle is different it will change the trail.
You can work this out before buying and fitting them!

Bottom Bracket Height

In general terms, to set the bike up for your size, the bike seat and your body will be a set distance from the center of the crank / bottom bracket.

If this is too high you will find it hard to touch the ground when stationary, your center of gravity will be high and the bike will be less stable.
Your body is also higher up causing more wind drag.

If this is too low the pedals may hit the ground particularly over rough ground in ruts and when cornering.

Again - somewhere in the middle is just perfect!

When you change forks on a bicycle or to a lesser extent change tyres the bottom bracket height will change.
Also I would speculate that a frame designed for road use will tend to have a smaller bottom bracket height and thus not have the ground clearance for off road use but do not have information to back this up.

Effective top tube, Wheelbase and Seat to Bottom Bracket

Reach and Stack

There is a great deal of information on the internet about setting up a bike to fit your body.
In very general terms, the size of the bike needs to scale up to each individuals body size.

Different manufacturers use different measurements to characterise size and fit:
Some manufacturers provide the 'effective top tube' measurement as most characterising overall size whilst others specify 'Reach' and 'Stack'.

You can use this website to work out what these are for your current bike to make comparisions. This is easier than measuring them directly as you can see from the diagram.

The wheelbase is also important and provides a guide to the overall size of the bicycle.
Changing the seat position, (sliding the seat forward / backwards), the handlebars and the length and height of the steering stem can
drastically effect how the bike feels and the pleasure of riding it. Compare your bikes with this website and analyse what works for you and what doesn't.

I am quite tall and my seat is quite high. My own perception is that if I ride a bike that has too short a wheelbase and effective top tube (or stack),
with a very high seat (and thus high center of gravity) when I go down even moderately steep hills offroad it feels like I will topple over the handlebars.
Over soft front suspension makes this effect worse. A longer effective top tube / stack and wheelbase and stiffer suspension solves this problem. Letting the seat down a bit offroad also helps!

If the wheelbase is too long the bicycle feels too big and unwieldy.

If the effective top tube / stack is too long your body is reaching too far forward to hold the bars. If it is too short you feel cramped when riding the bike.

The horizontal distance from the seat to the bottom bracket is very important. There is a great deal of information on the internet about how to set this up. Certain forms of racing
mandate minimum measurements from the front of the seat.

From personal experience if the horizontal distance from the seat to the bottom bracket is too large then my body has to bend more to hold the bars at a particular height.
It also feels to me that I can exert more pressure on the pedals if the seat is further forward than normal. The important issue here is what is right for you.
Compare your most comfortable bike with others by overlaying the graphic images and analyse what the differences are.

Whilst every effort has been made to ensure the accuracy of the information on this website, mistakes are possible therefore use it at your own risk!

Phil Barthram is a participant in the Amazon EU Associates Programme, an affiliate advertising programme designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.co.uk